The task of steering a beam of light is fundamental to many photonics applications such as light switching in a fiberoptic communications network, laser marking and material processing, laser printing, optical scanning and signaling, and other applications. One of the most common elements used to provide an optical beam steering function is a tiltable or rotatable mirror. A galvo-driven tiltable mirror, for example, is frequently used in laser printers. A rotatable mirror is often used in barcode scanners. A micro-electro-mechanical (MEMS) mirror is used in fiberoptic switches. When a mirror is rotated, the angle of incidence of the light beam on the mirror changes, which changes the angle of reflection and thus steers the light beam. Another way of interpreting the phenomenon of steering light with a tiltable mirror is to consider changes to the wavefront of a light wave caused by tilting a mirror the wave impinges on. A wavefront is a surface of constant phase of a light wave. It is known that, in an isotropic medium, a light wave tends to propagate in a direction perpendicular to its wavefront. Tilting of the mirror results in introducing a tilt into the wavefront of the reflected light wave and thus results in a change of direction of propagation of the wave.
Instead of a tilting mirror, an array of tunable optical phase delay elements may be used to effect, by generating a linear distribution of an optical phase delay across the surface of the array, a tilt on the wavefront of a monochromatic light wave and thus steer the light wave in a desired direction. Such a steering, which is sometimes called a “phased array beam steering”, can be used to control an angle of propagation of an optical beam represented by superposition of light waves traveling in a common direction. An array of deformable or displaceable MEMS elements, or an array of liquid crystal (LC) elements, disposed to interact with the wavefront of the light wave so as to cause a local delay, or retardation, of said wavefront, can be employed to introduce a controllable tilt in the wavefront of light waves impinging on the array and thus to steer the light waves in a desired direction. The mechanism of steering of a light wave by an array of tunable optical phase delay elements is somewhat similar to a mechanism of steering an electromagnetic pulse in a phased-array radar, wherein a controllable phase delay pattern is introduced into signals applied to individual electromagnetic emitters of the radar's phased array, so as to send the resulting electromagnetic pulse in a chosen direction.
In fiberoptic communication networks, it is a common technical problem to switch an optical signal at a particular wavelength from one fiber to another. An array of flat tiltable MEMS mirrors, or alternatively, an array of tunable LC polarization rotators, can be used as a switching element. For example, in U.S. Pat. No. 6,498,872 by Bouevitch et al., which is incorporated herein by reference, an optical configuration for a configurable add/drop multiplexer is described, wherein an array of LC elements is used to attenuate and, or switch optical signals at different wavelengths traveling in an optical fiber, by changing the polarization states of the optical signals at different wavelengths. Further, in U.S. Pat. No. 6,707,959 by Ducellier et al., which is incorporated herein by reference, a wavelength selective switch is described that uses an array of tiltable flat MEMS micro-mirrors to direct optical signals at different wavelengths into a particular of a plurality of output optical fibers, wherein the signals at different wavelengths are switched independently from each other.
One limitation of the wavelength selective switch of U.S. Pat. No. 6,707,959 is that an optical signal at a particular wavelength can only be switched into one output fiber at any moment of time. The reconfigurable add/drop multiplexer described in U.S. Pat. No. 6,498,872 can be used to split the optical power of an output signal between no more than two output optical fibers, because there are only two orthogonal states of polarization of a polarized light. A technology allowing simultaneous coupling of an optical signal into more than two optical waveguides, or, in general, into a selectable subset of a set of output optical waveguides, has some interesting applications. Such reconfigurable broadcasting fiberoptic modules can be used, for example, in “fiber-to-the-home” systems for delivering broadband Internet and, or high definition television services, carried by a single optical fiber, to many individual subscribers. A tiltable flat micromirror or a tunable polarization rotator technologies used in the devices of the abovementioned U.S. Pat. Nos. 6,498,872 and 6,707,959 cannot be readily employed for the purpose of reconfigurable broadcasting, because these technologies are not very suitable for splitting a light beam, in a reconfigurable manner, into a plurality of beams propagating in different directions. Advantageously, an array of tunable phase delay elements can be used to split and redirect a light signal consisting of a plurality of light waves, by properly modifying the wavefront of the light waves, so as to cause them to propagate in an arbitrarily selectable subset of a set of directions corresponding to a set of output fibers of a broadcasting optical switching device.
Spatial light modulators (SLMs) and, in particular, arrays of tunable phase delay elements have been employed as a switching elements in fiberoptic switching modules of the prior art. For example, in U.S. Pat. No. 7,397,980 by Frisken, which is incorporated herein by reference, a dual-source optical wavelength processor is described that uses a phased array for switching an optical signal at a particular wavelength, carried by an input fiber, into one of, or more than one of, output optical fibers. The switching function is performed by generating a linear distribution of optical phase delay across the surface of the array. The wavelength processor of Frisken comprises collimating optics, polarization manipulation optics, and a wavelength dispersing element such as a diffraction grating optically coupled to a prism, which is sometimes called a “grism”, for spreading optical signals at different wavelengths and polarizations across a single phased array. As a result of the spreading of the optical signals, the number of phase delay elements available for steering an individual light beam is much smaller than the total number of the elements in the array. The smaller the number of elements available for steering an individual light beam, the larger the diffraction sidelobes in the angular power distribution of the reflected light beam. Disadvantageously, the larger sidelobes create higher levels of an optical crosstalk.
The optical crosstalk in a fiberoptic network is highly undesirable, for the following reason. When a first optical signal at a wavelength λ1 is dropped by a wavelength selective switch at a particular location of the network, and another, second optical signal at the same wavelength λ1 is added at a downstream location, a residual first optical signal interferes coherently with the second optical signal at the downstream location, which leads to large fluctuations of an optical power level corresponding to low optical power, or a “zero” in a binary stream consisting of “ones” and “zeroes”, carried by the second optical signal at the same wavelength λ1. Because of the coherent nature of the interference, optical crosstalk in a wavelength selective switch can noticeably degrade performance of a fiberoptic communication link serviced by the switch, even at crosstalk levels as low as −35 dB.
The optical crosstalk problem was recognized in U.S. Pat. No. 6,975,786 by Warr et al., which is incorporated herein by reference, wherein an optical switch having two liquid crystal SLMs is described. In the switch of Warr et al., a light from an input fiber of an input fiber array diffracts on holograms displayed by the SLMs, and the diffracted light couples into a particular of an output fiber array. A crosstalk appears when a light that was intended to follow one path has a residual component that follows another path. According to Warr et al., the crosstalk can be reduced by selecting such set of holograms and such a set of output fiber locations where the optical power of the residual component of light is minimized. This is achieved by going through an iterative process of generating a set of N binary holograms for routing of light into one of N output fibers, calculating an angular distribution of optical power of diffracted light, and adjusting the physical locations of the input and the output fibers to minimize crosstalk into unintended fibers. Disadvantageously, the method of Warr et al. is computation-intensive; it requires the N holograms corresponding to a single input fiber of the input fiber array to be computed in advance and stored in a memory circuitry of the optical switch. Further, disadvantageously, due to optimizing relative fiber positions, the apparatus of Warr et al. is likely to contain output fiber and lenslet arrays with irregular pitch, which is impractical.
Accordingly, it is the goal of the present invention to provide a method for steering light using an array of tunable phase delay elements, wherein the optical power of light propagating in undesired directions is reduced, in comparison with the optical power of light diffracted from an array having the tunable phase delay elements driven so as to generate a linear optical phase delay distribution across the surface of the array. It is also the goal of the present invention to provide an optical switch having an optical signal broadcasting capability, wherein the optical crosstalk is reduced as compared to a crosstalk level in an optical switch having a linear optical phase delay distribution across the array of tunable phase delay elements.